1. Field of the Invention
The present invention relates to a dielectric-laminate type band-pass filter for use in a portable radio or the like having an operating frequency in the range of several hundred MHz to several GHz.
2. Description of the Prior Art
Conventional resonators are divided roughly into resonators using a strip line and resonators using a coil pattern. When producing a band-pass filter by using such resonators, plural resonators are coupled magnetically.
Examples of resonators using the strip line are a resonator of 1/2 wavelength whose line is open at both ends as shown in FIG. 18, and a resonator of 1/4 wavelength whose line is open at one end and shortcircuited at the other end as shown in FIG. 19.
An example of a resonator using the coil pattern, as shown in FIG. 20, is one in which a spiral-shaped coil pattern 201 and an earth pattern 203 are formed on both sides of a dielectric layer 202 which is clamped therebetween.
In the band-pass filter using the above-mentioned conventional resonators, however, such problems as shown hereinafter were encountered respectively.
[1] band-pass filter using the strip line
(a) A resonator having a resonance frequency of 2 to 3 GHz becomes substantially large. In particular, in the band-pass filter having a construction in which plural resonators are connected, it becomes considerably large. This is due to the following reasons.
The lengths L.sub.10, L.sub.11 of the strip line are determined as shown in Equation 1 (resonator of 1/2 wavelength) and Equation 2 (resonator of 1/4 wavelength). ##EQU1##
Where, .lambda.; wavelength, .epsilon.; dielectric constant of dielectric-laminate sheet.
At present, a dielectric constant of dielectric-laminate sheet which can be co-fired with silver or copper and has a good temperature characteristic can not be made so large, only about .epsilon..apprxeq.10. Thus, in the above Equations 1 and 2, when the resonance frequency is 3 GHz, and supposing .epsilon.=10, L.sub.10 =15.8 mm and L.sub.11 =7.9 mm, which are very long, thus resulting in a large resonator (band-pass filter) as stated above.
(b) in the band-pass filter, it is desirable to adjust the input/output impedances depending on the apparatus into which it is incorporated (to adjust and thereby match the impedances of the band-pass filter and the apparatus). However, in case of the strip-line type, since the input/output impedances have peculiar values for every strip line, it is impossible to adjust and match, even by changing the input/output connection position of the strip line.
[2] band-pass filter using the coil-pattern
Since the coil pattern has a spiral shape, magnetic fluxes influence one another between the adjoining patterns, thus interfering with electric current flow. Therefore, a substantial resistance increases and the Q of the filter becomes lower.
For example, in FIG. 20, since the electric current flows in the same direction (in a direction D in FIG. 20) in a pattern piece 201a and a pattern piece 201b, magnetic fields cancel each other to cause the magnetic flux to become coarse, and consequently electric current flow is disturbed and the substantial resistance increases.
The increase in resistance causes that an insertion loss of the band-pass filter becomes larger as a result of the reduction in Q.